1. Field of the Invention
The present invention relates to a current/charge-voltage converter that uses an integrating circuit and in particular, to a current/charge converter that comprises a means for resetting the charge that has accumulated in the integrating capacitor to a specific voltage.
2. Discussion of the Background Art
Current/charge-voltage converters that have integrating circuits are used in current measuring devices, charge measuring devices, and other devices. These are circuits wherein, as shown in the integrating circuit 1 of FIG. 2, an integrating capacitor 11 is connected between the inverted input terminal and output terminal of an operational amplifier 10, integrating capacitor 11 is charged by current from the current source of device under test 3, the amount of charge is measured by measuring the integrated voltage V, and the amount of current is measured by finding the change in integrated voltage V. The current/charge-voltage converter of the present invention encompasses both current-voltage conversion, as well as charge-voltage conversion.
When plural charges or currents are measured using the integrating circuit in FIG. 2, it is necessary to reset capacitor 11 so that capacitor 11 does not become saturated. In general, this reset operation involves discharging the charge that has accumulated in capacitor 11 and resetting the output voltage Vo to 0 V, and it becomes necessary to reset the voltage between the two terminals of capacitor 11 to a specific voltage when the polarity of the measured signals is known in advance, when there is an output voltage limit that we can measure with a high degree accuracy, and the like.
The technology cited in JP (Kokai) 3[1991]-200,121 is a current/charge-voltage converter having the function of resetting the output voltage Vo to a specific voltage. This circuit will be generally described based on the schematic representation in FIG. 2. The current-voltage conversion current can be divided into an integrating circuit 1 and a reset circuit 2. A current source of the device under test 3 is connected to integrating circuit 1. On the other hand, reset circuit 2 is connected between the two terminals of integrating capacitor 11 and has the function of resetting the voltage of the capacitor. Reset circuit 2 comprises an FET switch 23, a control signal source 25 of switch 23, an operational amplifier 27 connected to switch 23, and a signal source 28 that applies the reset voltage. When the output of control signal source 25 is 0 V, the gate voltage of FET 23 becomes 0 V; therefore, the drain-source is in a disconnected state and reset circuit 2 is in a non-operating state.
On the other hand, when a positive voltage is applied to control signal source 25, the drain-source of FET 23 is in a connected state. The potential of the noninverted input terminal of operational amplifier 27 and junction A becomes the same voltage as the inverted input terminal at this time. Therefore, the noninverted input terminal and junction A become the same potential as the output voltage V2 of signal source 28. In addition, the output voltage of operational amplifier 27 and the potential of junction B become 0 V because they are connected to the inverted input terminal of an operational amplifier 10. Moreover, both terminals of capacitor 11 are reset to V2, which is the potential difference between junctions A and B.
Thus, the circuit in FIG. 2 can charge capacitor 11 to a specific voltage. However, there is a floating capacitor 26 of capacitance value C1 between the control terminal (gate in FIG. 2) and the terminal to be controlled (source in FIG. 2) of FET switch 23, as well as analog switches, relays, and other types of switches. Therefore, when the voltage of control signal source 25 changes, current flows to the source terminal. That is, current flows to the source terminal at the instant when switch 23 is turned off. There is a problem in that because this current flows into capacitor 11, the output voltage Vo (the voltage between the two terminals of capacitor 11) changes after switch 23 has been turned off and becomes a potential that is different from the reset voltage V2. In particular, a capacitor 11 with a small capacitance is used for improved accuracy in devices that measure microcurrent (or microcharge); as a result, there are large changes in the output voltage Vo with the source current that flows when switch 23 is turned off.